In the present day, ion implanters are often constructed to optimize implantation according to a specific set of applications. In some applications, for example, it may be useful to increase beam current to increase throughput of substrates to be implanted. Some beamline ion implanters may be employed to implant substrates over a range of energies, such as between 1 keV and 300 keV. This capability provides flexibility for processing substrates such as silicon wafers, for example, in circumstances where the silicon wafers are scheduled for various implants at differing ion energies. In order to define an implant energy, an ion beam may be subject to acceleration and deceleration by various components in a beamline ion implanter between an ion source and substrate to be implanted.
In an ion implanter used for ion implantation over a wide range of energies, such as between 1 keV and 300 keV, it may be convenient to extract ions from an ion source at a target energy, such as 50 keV, regardless of the target ion implant energy to be imparted to ions implanted into a substrate. This practice may ensure adequate beam current is extracted from an ion source, even for low energy implantation, since the extraction current I has a dependence upon extraction voltage V, wherein I is proportional to V3/2 in accordance with the Child-Langmuir law. Accordingly, an ion beam having an initial ion energy of, for example, 50 KeV or greater may be received by an acceleration/deceleration column (accel/decel column) configured to adjust beam potential (beam voltage) of the ion beam in order to impart a target beam energy to the ion beam. The accel/decel column may include multiple electrodes configured to adjust beam potential and shape the ion beam.
In order to ensure stable operation, the electrode separation of various electrodes in an accel/decel column may be set at a adequate distance to prevent electrical breakdown or arcing, a process tending to increase at higher voltages. Accordingly, the electrode separation between a ground electrode and focus electrode of an accel/decel column may be set to avoid arcing at a highest operating voltage for an ion implanter, such as 300 kV. This practice may result in undesirably lower current extracted for lower ion energies. It is with respect to these and other considerations the present improvements have been needed.